LibWeb: Return undefined from generated setters, not an empty value

These now crash as VM::call() uses ThrowExceptionOr<T>, which refuses to
hold an empty JS::Value as its non-exception result.
We only need to return an empty value when should_return_empty() says
so for the return value of throw_dom_exception_if_needed().

Co-authored-by: Luke Wilde <lukew@serenityos.org>

1 files changed, 5 insertions, 2 deletions

diff --git a/Meta/Lagom/Tools/CodeGenerators/LibWeb/WrapperGenerator.cpp b/Meta/Lagom/Tools/CodeGenerators/LibWeb/WrapperGenerator.cpp
index 9e48eb8ec8..c42dcab99a 100644
--- a/Meta/Lagom/Tools/CodeGenerators/LibWeb/WrapperGenerator.cpp
+++ b/Meta/Lagom/Tools/CodeGenerators/LibWeb/WrapperGenerator.cpp
@@ -1739,12 +1739,15 @@ JS_DEFINE_NATIVE_FUNCTION(@prototype_class@::@attribute.setter_callback@)
                 }
             } else {
                 attribute_generator.append(R"~~~(
-    [[maybe_unused]] auto retval = throw_dom_exception_if_needed(vm, global_object, [&] { return impl->set_@attribute.name:snakecase@(cpp_value); });
+    auto result = throw_dom_exception_if_needed(vm, global_object, [&] { return impl->set_@attribute.name:snakecase@(cpp_value); });
+
+    if (should_return_empty(result))
+        return JS::Value();
 )~~~");
             }
 
             attribute_generator.append(R"~~~(
-    return {};
+    return JS::js_undefined();
 }
 )~~~");
         }